Great Blue Hole, Belize.

This incredible geographical phenomenon known as a blue hole is situated 60 miles off the mainland of Belize .There are numerous blue holes around the world but none as stunning as this one.
A blue hole is a submarine cave (inland) or underwater sinkhole. They are also called vertical caves. Out of the numerous blue holes, the best known examples can be found in Belize, the Bahamas, Guam, Australia (in the Great Barrier Reef), and the Red Sea.

The Great Blue Hole is a large underwater sinkhole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll 70 kilometres (43 mi) from the mainland and Belize City. The hole is circular in shape, over 300 metres (984 ft) across and 124 metres (407 ft) deep.[1] It was formed during several episodes of Quaternary glaciation when sea levels were much lower - the analysis of stalactites found in Great Blue Hole shows that formation has taken place 153,000; 66,000; 60,000; and 15,000 years ago.[2] As the ocean began to rise again, the caves were flooded.[3] The Great Blue Hole is a part of the larger Belize Barrier Reef Reserve System, a World Heritage Site of the United Nations Educational, Scientific and Cultural Organization (UNESCO).